A liquid crystal display device uses a backlight unit to illuminate its liquid crystal panel (LCD) by radiating light from the rear side of the liquid crystal panel.
The backlight unit is configured using such parts as a light source for illumination, a light guide plate for dispersing light emitted from the light source to irradiate the liquid crystal display panel therewith, and a prism sheet and a diffusion sheet for rendering the light radiated from the light guide plate uniform.
Currently, large liquid crystal televisions predominantly use a backlight unit of a type called a direct illumination type (see JP 05-4133 U, for example). A backlight unit of that type has cold cathode tubes provided as light sources on the rear side of the liquid crystal display panel, the inside providing a white reflective surface, to ensure a uniform light amount distribution and necessary brightness. To achieve a uniform light amount distribution with this type of backlight unit, however, the liquid crystal display panel needs to have a thickness of about 30 mm in a direction perpendicular to the panel as dictated by the principle.
Recent years have been seeing demands for thinner, less power consuming, or larger liquid crystal display devices. However, the backlight unit of direct illumination type mentioned above presented a limit to how thin the thickness could be made because of unevenness in light amount distribution that occurred when the thickness of the light guide plate was reduced to 10 mm or less. Thus, light guide plates of various configurations have been proposed to achieve thinner design, reduced power consumption, and larger dimensions (see JP 09-304623 A, JP 10-133027 A, and JP 2001-42327 A).
For example, JP 09-304623 A discloses a planar light source device (backlight unit) formed such that fluorescent lamps are embedded in the parallel grooves formed in a light guide plate having a substantially rectangular shape, reflective sheets are disposed on the rear surface of the light guide plate, and a transmitted light amount correction sheet, a light diffusion plate, and a prism sheet are laid on one another on the light emitting plane of the light guide plate.
JP 10-133027 A describes a light guide unit (light guide plate) comprising a recess having a parabolic shape in cross section parallel to a widthwise direction of the recess for accommodating a light source, the major axis of the parabolic shape lying in the direction of depth of the recess, in order to obtain a backlight unit that, with a high light use efficiency and, hence, high brightness, makes it possible to provide a liquid crystal display device having a thinned frame and a reduced thickness.
The light guide plates described in JP 09-304623 A and JP 10-133027 A aim to achieve some of a thinner design, a reduction in size and weight, less power consumption, and reduced manufacturing costs for liquid crystal display devices. In both of these references, the light guide plate has one or more grooves formed near the center thereof to accommodate a rod-type light source therein, and the thickness of the plate preferably decreases gradually from the groove toward the end surfaces to achieve a thinner design.
JP 2001-42327 A achieves a large, high-brightness, high-uniformity rear surface illumination using light guide plates arranged in juxtaposition and a given number of linear light sources provided between the light guide plates to improve a liquid crystal backlight unit so it provides a large liquid crystal display surface for wall-mounted televisions.
The liquid crystal display devices disclosed in JP 05-4133 U, JP 09-304623 A, JP 10-133027 A, and JP 2001-42327 A use cold cathode tubes as light sources for the backlights. In recent years, a backlight using LEDs (light emitting diodes) in place of cold cathode tubes has been proposed. JP 09-259623 A, for example, discloses an invention related to an LED light source module wherein at least one side of a light guide plate in the form of a plate is adapted to serve as light source mounting side, a proper number of light admitting portions in the form of recesses are provided on the light source mounting side, and LED lamps are respectively disposed opposite the light admitting portions. In Patent Document 5, the light admitting portions are formed into recesses on the light source mounting side such that each of the inner sides, i.e., the adjacent sides of the recesses forming a pair is formed by cutting into the light source mounting side at an angle closer to normal and each of the outer sides of the recesses forming the pair is formed by cutting into the light source mounting side at an angle closer to parallel. Between the pair of the light admitting portions is provided a reflective surface having a concave surface.
JP 2001-110223 A discloses an electrooptical device comprising a first light guide unit in the form of a plate disposed opposite the front side of an electrooptical panel, a second light guide plate extending along a lateral end surface of the first light guide unit, point light sources for causing light to enter the second light guide unit through the ends portions thereof, and incidence area limiting means for preventing light from entering the second light guide unit through the end portions of the second light guide unit. The second light guide unit used therein is a columnar (prismatic) translucent resin mold, adjacent both ends of which point light sources are provided. Light from the point light sources is led into the second light guide unit and emitted through the side walls of the light guide unit.
JP 2000-268622 A discloses a planar lighting device using a light source configured by a light guide unit made of a translucent material and a point light source provided adjacent at least one end of the light guide unit. The light guide unit has a rectangular or circular cross section that decreases with the increasing distance from the point light source and is disposed adjacent a lateral end surface of a rectangular light guide plate.
Patent Document 1: JP 05-4133 U
Patent Document 2: JP 09-304623 A
Patent Document 3: JP 10-133027 A
Patent Document 4: JP 2001-42327 A
Patent Document 5: JP 09-259623 A
Patent Document 6: JP 2001-110223 A
Patent Document 7: JP 2000-268622 A